Temperature‐Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non‐Fluorinated Ionic Liquids

Abstract

Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), we investigate the topmost nanometers of various binary ionic liquid (IL) mixtures at differentt emperatures in the liquid state. The mixtures consist of ILs with the same [PF 6 ] À anion but two different cations, namely 3methyl-1- (3,3,4,4,4-pentafluorobutyl)imidazoliumh exafluorophosphate, [PFBMIm][PF 6 ], and 1-butyl-3-methylimidazolium hexafluorophosphate, [C 4 C 1 Im][PF 6 ], with 10, 25, 50 and 75 mol %c ontento f[ PFBMIm][PF 6 ]. We observe ap referential enrichmento ft he fluorinated chain in the topmostl ayer, relative to the bulk composition, which is mostp ronounced for the lowest content of [PFBMIm][PF 6 ]. Upon cooling the mixtures stepwise from 95 8Cu ntil surfacec harging effects in XPS indicate solidification,w eo bserve ap ronouncedi ncreasei ns urfacee nrichmento ft he fluorinated chain with decreasing temperature in the liquid state. In contrast to the mixtures with lower[ PFBMIm] [PF 6 ]c ontents, cooling the 75 mol %m ixture additionallys hows an abrupt decrease of the fluorinated chain signal before complete solidification occurs, whichisa ssigned to partial precipitation effects.[a] B.

Ultrathin ionic liquid films on metal surfaces: adsorption, growth, stability and exchange phenomena

“…The broad range for the WL desorption could also be a sign of decomposition of the [82]. This observation indicates that the seemingly significant structural variation at the cation of these two ILswhich has a profound effect on the interface composition and enrichment behavior in mixed films [82,145] (see below)has only little effect on the multilayer desorption. For the WL of [PFBMIm][PF 6 ] on Ag(111), XPS clearly indicates decomposition of the IL above 420 K [82], in contrast to [C 8 C 1 Im][PF 6 ] on Ag (111), where no signs of decomposition are observed upon thermal desorption [83].…”

Surface Tension and Viscosity of Binary Mixtures of the Fluorinated and Non-fluorinated Ionic Liquids [PFBMIm][PF6] and [C4C1Im][PF6] by the Pendant Drop Method and Surface Light Scattering

“…This is reflected in deviations of the experimental data from linear mixing rules. Deviations are found especially for the surface tension as the surface-near region can show a composition which is significantly different from that in the bulk [12,30]. Though very speculative, it is common practice that thermophysical property data of ILs or IL mixtures are used to draw conclusions about the fluid structure.…”

“…The latter are preferentially enriched at the IL surface given their relatively low cohesive energy [10,16,32]. By including fluorinated chains in the anions or cations of pure ILs, experiments [30,33] and molecular dynamics (MD) simulations [9,17,28] showed that the bulk and surface characteristics are altered. In the liquid bulk, the polar and nonpolar colloid-like domains are accompanied by nonpolar "fluorous" domains containing the fluorinated chains [9,28,33].…”

“…To investigate whether such surface enrichment effects are also present in mixtures of fluorinated and non-fluorinated ILs, Heller et al [30] have recently applied angle-resolved X-ray photoelectron spectroscopy (ARXPS). With this method, the composition and molecular arrangement at the surface-near region with an information depth of about (1.0 to 1.5) nm can be analyzed under ultra-high vacuum conditions; see, e.g., Refs.…”

“…With this method, the composition and molecular arrangement at the surface-near region with an information depth of about (1.0 to 1.5) nm can be analyzed under ultra-high vacuum conditions; see, e.g., Refs. [30,[34][35][36]. For binary mixtures of the non-fluorinated IL [C 4 C 1 Im][PF 6 ] (1-butyl-3-methylimidazolium hexafluorophosphate) and the structurally similar fluorinated IL [PFBMIm][PF 6 ] (3-methyl-1-(3,3,4,4,4-pentafluorobutyl) imidazolium hexafluorophosphate) with a partial fluorination in the cationic alkyl chain, a pronounced enrichment of the fluorinated chain at the surface was observed [30], which is in accordance with the aforementioned findings for pure ILs [17] and with those reported for another binary mixture of fluorinated and nonfluorinated ILs [12].…”